Got an old MacMini that I repurposed for a specific test purpose (hardware that was to be discarded but serve the intended purpose)

Installed Debian 9.3 (Well the current release) and everything is fine, booting and working correctly. However, as soon as I try to boot it up without a display connected (headless), it simply won't start/work.

Well, hard to say, but no network connectivity, so I'm not sure if it boot entirely or stops somewhere along the process. Obviously, cannot access it via SSH, and plugging a monitor afterward does not work (no display). I find myself surprised that it cannot be booted without any kind of monitor connected to it.

Googling the topic ended up with a lot of chat regarding changing the GRUB setup, changing xorg.conf file, installing dummy display drivers, xvnc servers and what else, and so far, nothing has been successful (and I kept in mind that almost everything was targeted toward Ubuntu, but technically should apply as well to Debian to some extent).

Best place to find info on PPC is the mailing list. Check the wiki's of Debian and Ubuntu as well. The Ubuntu one will probably be more up to date. You could also try the irc channels and if that fails try similar searches on Gentoo. Which is probably the place to best find info on PPC. It's a small world so most fixes or issues are shared.